The invention relates to a two-step process for the preparation of organic compounds from a mixture of hydrogen and carbon monoxide.
Organic compounds, such as aromatic hydrocarbons, paraffinic hydrocarbons and oxygen-containing compounds, particularly methanol, ethanol and dimethyl ether, can be prepared by catalytic conversion of H.sub.2 /CO mixtures.
Aromatic hydrocarbons may be prepared, for instance, by contacting a H.sub.2 /CO mixture having a H.sub.2 /CO molar ratio lower than 2.0 with a bifunctional catalyst combination comprising one or more metal components having catalytic activity for the conversion of a H.sub.2 /CO mixture into acyclic hydrocarbons and/or acyclic oxygen-containing organic compounds and a crystalline metal silicate capable of catalyzing the conversion of acyclic hydrocarbons and acyclic oxygen-containing organic compounds into aromatic hydrocarbons, with the understanding that if the H.sub.2 /CO mixture has a H.sub.2 /CO molar ratio lower than 1.5, a trifunctional catalyst combination is used which comprises one or more metal components having catalytic activity for the conversion of a H.sub.2 /CO mixture into acyclic hydrocarbons and/or acyclic oxygen-containing organic compounds, one or more metal components having CO-shift activity and the crystalline metal silicate mentioned hereinbefore. An investigation into this process has shown that it has two drawbacks. In the first place, when using space velocities acceptable in actual practice, the conversion of the H.sub.2 /CO mixtures leaves much to be desired. Further, the process yields a product consisting substantially of hydrocarbons having at most 12 carbon atoms in the molecule, and but a very small proportion of hydrocarbons having more than 12 carbon atoms in the molecule.
Continued research into this process has shown that the two drawbacks mentioned hereinabove can be overcome by contacting, in a second process step, hydrogen and carbon monoxide present in the reaction product of the process, together with other components of that reaction product, if desired, with a catalyst containing one or more metal components having activity for the conversion of a H.sub.2 /CO mixture into paraffinic hydrocarbons, the metal components having been chosen from the group formed by cobalt, nickel and ruthenium, provided that the feed for the second step is made to have a H.sub.2 /CO molar ratio of 1.75-2.25. What is achieved in this way is not only that, at space velocities acceptable in actual practice, a very high conversion of the H.sub.2 /CO mixture is obtained, but also that a considerable proportion of the reaction product consists of hydrocarbons of having more than 12 carbon atoms in the molecule. The reason of the second step of the process is that the CO present in the feed for the second step as much as possible is converted into paraffinic hydrocarbons. To this end the H.sub.2 /CO molar ratio of the feed for the second step should be 1.75-2.25. In some cases, for instance when a H.sub.2 /CO mixture with a high H.sub.2 /CO molar ratio is available for the process, the first step may yield a reaction product which has a H.sub.2 /CO molar ratio of 1.75-2.25 and is suitable without further treatment for use as the feed for the second step. In most cases, however, the first step will yield a product having a H.sub.2 /CO molar ratio lower than 1.75 and special measures will have to be taken to ensure that the feed which is contacted with the catalyst in the second step has the desired H.sub.2 /CO molar ratio of 1.75-2.25.
An investigation has been made into six measures which might be suitable for the purpose. The measures examined were the following:
(1) Water may be added to the feed for the first step and the trifunctional catalyst combination mentioned hereinbefore may be used in the first step. Under the influence of the CO-shift activity of the trifunctional catalyst combination the added water reacts with CO from the feed to form a H.sub.2 /CO.sub.2 mixture. This measure has the drawback that the activity of the catalyst combination is adversely affected both by the presence of the added water and by the presence of the carbon dioxide produced. PA1 (2) The feed for the first step, together with water, may be subjected to CO-shift in a separate reactor. Since the CO-shift is an equilibrium reaction, the reaction product will contain unconverted water. Besides, the reaction product will contain carbon dioxide formed. As already stated in the discussion of the first measure, water and carbon dioxide have an adverse effect on the activity of the catalyst combination in the first step. Since, in view of the high cost involved, the removal of water and carbon dioxide from the CO-shift reaction product is not suitable for use on a technical scale, this second measure has the same drawback as that mentioned for the first measure. PA1 (3) From the feed for the second step having a low H.sub.2 /CO molar ratio so much CO may be separated that the desired H.sub.2 /CO molar ratio is attained. In view of the high cost attending separation of CO from the feed for the second step, this measure is not suitable for use on a technical scale. PA1 (4) To the feed for the second step having a low H.sub.2 /CO molar ratio so much H.sub.2 may be added that the desired H.sub.2 /CO molar ratio is attained. Since the hydrogen required is not formed in the process, it will have to be supplied to the process from outside, which is a costly affair and, therefore, is not suitable for use on a technical scale. PA1 (5) Water may be added to the feed for the second step and in the second step a bifunctional catalyst combination may be used which, in addition to metal components having catalytic activity for the conversion of a H.sub.2 /CO mixture into paraffinic hydrocarbons, contains one or more metal components having CO-shift activity. The bifunctional catalyst combination used in the second step is usually composed of two separate catalysts, which, for convenience, will be referred to as catalysts A and B. Catalyst A is the CO-, Ni- or Ru-containing catalyst and catalyst B is the CO-shift catalyst. For the use of a bifunctional catalyst combination in the second step, the following four embodiments may be considered. PA1 (5a) Carrying out the second step in a reactor containing a physical mixture of catalysts A and B. PA1 (5b) Carrying out the second step in a reactor containing a fixed catalyst bed consisting of a layer of catalyst B, followed by a layer of catalyst A, both catalysts being used at the same temperature. PA1 (5c) A procedure substantially corresponding with that described under (5b), but in which catalyst B, is used at a higher temperature than catalyst A. PA1 (5d) Carrying out the second step in two separate reactors, the first containing catalyst B, the second catalyst A, and the temperature used in the first reactor being higher than that used in the second reactor. PA1 (6) So much of a hydrogen-rich H.sub.2 /CO mixture may be added to the feed for the second step having a low H.sub.2 /CO molar ratio, that the desired H.sub.2 /CO molar ratio is reached. On the face of it, this measure, which is related to that mentioned under (4), seems unfit for use on a technical scale as well, as the required hydrogen-rich H.sub.2 /CO mixture is not formed in the process and has to be supplied to the process from outside. A favorable circumstance, however, lies in the fact that in the two-step process a low-hydrogen H.sub.2 /CO mixture is available as the feed for the first step. By separating a portion of this low-hydrogen H.sub.2 /CO mixture and subjecting it to CO-shift, a reaction product having a high H.sub.2 /CO molar ratio can be prepared in a simple way. In addition to hydrogen and carbon monoxide, this reaction product will contain unconverted water and carbon dioxide formed. Since the activity of the catalyst used in the second step of the process, in contrast to that of the catalyst combination in the first step, is hardly susceptible to the presence of water and carbon dioxide in the feed, this reaction mixture can be used as mixing component for the feed for the second step without water and dioxide having to be removed. In view of the drawbacks, described under 5, connected with the application of a low-temperature CO-shift to low-hydrogen H.sub.2 /CO mixtures, only a high-temperature CO-shift is eligible for the present purpose. The present patent application relates to the application of a CO-shift, at a temperature above 325.degree. C., to a low-hydrogen H.sub.2 /CO mixture which has been separated from the feed for the first step of the two-step process described hereinabove and to the use of the hydrogen rich H.sub.2 /CO mixture prepared as a mixing component for the feed for the second step of the process.
Each of these embodiments has its drawbacks, which have to do with the type of CO-shift catalyst to be used. On the basis of the temperatures at which they are active, CO-shift catalysts can be divided into two groups, viz. "high-temperature CO-shift catalysts" (active at temperatures of about 325.degree.-500.degree. C.) and "low-temperatures CO-shift catalysts" (active at temperatures of about 175.degree.-250.degree. C.). Low-temperature CO-shift catalysts are particularly suitable for use with H.sub.2 /CO mixtures already having a high H.sub.2 /CO molar ratio, where a low conversion is sufficient to attain the purpose in view. Such H.sub.2 /CO mixtures may very suitably be prepared from H.sub.2 /CO mixtures having a low H.sub.2 /CO molar ratio, by subjecting them to a high-temperature CO-shift. For attaining a high conversion in the case of H.sub.2 /CO mixtures having a low H.sub.2 /CO molar ratio (as described in the case of the feed for the second step of the process) the low temperature CO-shift catalysts are not very suitable, since, at the desired high conversion level, they are deactivated rapidly and also because at the low temperatures used, they tend to form methanol from low-hydrogen H.sub.2 /CO mixtures. High-temperature CO-shift catalysts, when applied to low-hydrogen H.sub.2 /CO mixtures, give a high conversion without being subject to rapid deactivation, and at the high temperatures used they show no tendency towards the formation of methanol. Since the temperature at which catalyst A is used in the second step of the process should be lower than 325.degree. C., only a low-temperature CO-shift catalyst is eligible as catalyst B, when a bifunctional catalyst combination is to be used in the way described under (5a) and (5b). As stated hereinabove, this has serious disadvantages in view of rapid deactivation and undesirable methanol formation. The embodiments mentioned under (5c) and (5d) offer the possibility of using a high-temperature CO-shift catalyst as catalyst B, but this involves another drawback connected with the composition of the reaction product from the first step. This product usually contains a certain percentage of lower olefins. Separation of these lower olefins from the reaction product of the first step cannot be considered for use in a technical scale in view of the high cost involved. This means that the feed for the second step will, in addition to hydrogen and carbon monoxide, as a rule contain lower olefins. These lower olefins often cause rapid deactivation of the high-temperature CO-shift catalyst.
As was remarked hereinbefore, catalytic conversion of H.sub.2 /CO mixtures can be used to prepare not only aromatic hydrocarbons, but also very suitable paraffinic hydrocarbons and oxygen-containing organic compounds.
Paraffinic hydrocarbons may be prepared, for instance, by contacting a H.sub.2 /CO mixture having a molar ratio below 2.0 with an iron-containing bifunctional catalyst or catalyst combination which, in addition to activity for the conversion of a H.sub.2 /CO mixture into, substantially, paraffinic hydrocarbons, has CO-shift activity. An investigation into this process has shown that the use of high space velocities presents difficulties. When the process is used for the conversion of H.sub.2 /CO mixtures having a H.sub.2 /CO molar ratio below 1.0, the stability of the bifunctional catalyst or catalyst combination leaves much to be desired. When the process is used for the conversion of H.sub.2 /CO mixtures having a H.sub.2 /CO molar ratio between 1.0 and 2.0, the conversion attained is low.
Oxygen-containing organic compounds may be prepared, for instance, by contacting a H.sub.2 /CO mixture having a H.sub.2 /CO molar ratio below 2.0, with a catalyst containing one or more metal components with catalytic activity for the conversion of a H.sub.2 /CO mixture into oxygen-containing organic compounds. A drawback to these reactions is the fact that they are highly limited thermodynamically, so that a considerable proportion of the H.sub.2 /CO mixture is not converted. According as higher space velocities are used, the conversion obtained are lower.